What is heavey water?

There are two types of heavy water. The first is where the hydrogen atom is replaced by the Deuterium atom, which contains a neutron. (Thus the atom for practical purposes) is twice as heavy. There is another similar form of hydrogen atom which contains two neutrons, and consequently weighs three times as much as the ordinary Hydrogen. Tritium has a half life of just over 12 years. Deuterium is stable.

If either of these hydrogen isotopes is used to make water, the water is heavier than usual. Commonly differentiated as 'Deuterated water' and Tritiated water'

[There is actually a third variety of heavy water in which 18O is used instead of 16O. This variety is 3H2 18O. Used in the production of positrons for medical use.]